1. Field of the Invention
This invention relates to a semiconductor device and particularly to a structure of a capacitor part using a metal oxide film.
2. Description of the Prior Art
In semiconductor devices, such as a dynamic random access memory, which is provided with a capacitor part as a structural element, integration density is yearly increased. With the increase of the integration density, the area which the capacitor occupies becomes smaller and a capacitor insulating film having a high specific dielectric constant has been needed. For this capacitor insulating film having the high specific dielectric constant, to use a film of a dielectric material, such as Ta.sub.2 O.sub.5, TiO.sub.2, Nb.sub.2 O.sub.5, HfO.sub.2, ZrO.sub.2 or the like, has been tried.
As the structure using the above dielectric film, there is a structure in which the dielectric film, for example the Ta.sub.2 O.sub.5 film, is formed on the first electrode of a silicon substrate, a polycrystal silicon or the like. Further the second electrode of a polycrystal silicon or the like is formed on the Ta.sub.2 O.sub.5 film thereon. In this structure, however, there are several defects that, when the Ta.sub.2 O.sub.5 film is formed, the silicon substrate or the like of the first electrode is oxidized to form a silicon oxide film, whereby the value of capacitance is reduced and in I-V characteristics are dependent on a polarity of a gate voltage. In addition, in a heat treatment after forming the second electrode, there is a defect that the polycrystal silicon of the second electrode reacts with Ta.sub.2 O.sub.5 and thus a leakage current is increased.
In the past, in order to obviate the above first defects, there has been proposed a method wherein a thin silicon nitride film is formed on a silicon substrate or polycrystal silicon and thereafter a Ta.sub.2 O.sub.5 film is formed thereon (e.g. H. Shinriki et al., IDEM Tech. Dig. P684, 1986). Also, in order to obviate the above second defect, there has been proposed a structure wherein a silicon nitride film or a silicon oxide film is sandwiched between a Ta.sub.2 O.sub.5 film and the second electrode (e.g. H. Shinriki et al., Tech. Res. Report of Electron Information Communication Society, SDM88-44, P25 (1988)).
In the prior capacitor structure as mentioned above, however, the silicon nitride film is sandwiched between the Ta.sub.2 O.sub.5 film and the first electrode or the silicon nitride film or silicon oxide film is sandwiched between the Ta.sub.2 O.sub.5 film and the second electrode. Thus, there is another defect that the capacitance value is reduced as compared with that of a single layer of the Ta.sub.2 O.sub.5 film.